Photovoltaic solar cells are semiconductor devices which convert sunlight into electricity. Solar cells based on crystalline silicon offer the advantage of high performance and stability. The principal barrier to expanded utilization of silicon solar cells for electric power generation is the present high cost of the solar cells.
In conventional solar cells based on single crystal or large grain polycrystalline silicon ingot processes, the major cost factor is determined by the requirement of sawing ingots into wafers. Sawing is an expensive processing step, and furthermore results in the loss of approximately half the costly ingot material as silicon dust. The problem to be solved requires the development of a low-cost process, that efficiently employs low-cost materials while maintaining solar cell performance.
The solution to the problem requires the achievement of a process that is controllable, has high areal throughput, and generates material with adequate crystalline morphology. Prior art includes several processes which either effectively achieve controlled growth, or high areal throughput of silicon sheet or ribbons. All these approaches eliminate the costly process of sawing large areas to create wafers from ingots. For example, publications by Hopkins (WEB), Ettouney, et al. (EPG), Gutler (RTR) and Eyer, et al. (SSP) describe processes that achieve controlled polycrystalline growth of grains greater than 1 mm in size at low linear speeds (and consequently low areal generation rates). Common to these sheet manufacturing processes is the fact that the sheet pulling direction and the direction of grain growth are collinear. All of these processes employ a large temperature gradient (&gt;500 degrees Centigrade per centimeter) along the sheet pulling direction. This gradient is necessary to achieve the practical linear sheet pulling velocity (typically less than 2 cm/min), but also introduces large thermal-induced stresses. In many cases these stresses limit the achievable practical sheet width by causing sheet deformations which make solar cell fabrication untenable. Thermal stresses can also create crystalline defects which limit solar cell performance. Each of these processes attempts to achieve grain sizes that are as large as possible in order to avoid the deleterious effects of grain boundaries on solar cell performance.
Another set of processes has been developed that can achieve high areal throughput rates. For example, publications by Bates, et al. (LASS), Helmreich, et al. (RAFT), Falckenberg, et al. (S-Web), Hide, et al. (CRP) Lange, et al. (RGS) and Hall et al. (SF) describe processes that achieve polycrystalline sheet growth with grain sizes in the 10 microns to 3 mm range at high linear rates (10 to 1800 cm/min). Typically, these processes have difficulty maintaining geometric control (width and thickness) (e.g. LASS, RAFT, RGS), and/or experience difficulty with contamination of the silicon by the contacting materials (e.g. RAFT, S-Web, CRP). The process of Hall, et al. (U.S. Pat. Nos. 5,336,335 and 5,496,416) effects geometric control and minimizes the contact of the growing silicon with deleterious materials. Common to these sheet growth processes is the fact that the sheet pulling direction and the direction of crystalline grain growth are nearly perpendicular. It is this critical feature of these processes that allows the simultaneous achievement of high linear sheet pulling velocities and reduced crystal growth velocities. Reduced crystal growth velocities are necessary for the achievement of materials with high crystalline quality.
The prior art regarding the fabrication of solar cells from polycrystalline silicon materials requires that the grain size be greater than 1.0 mm. This requirement on grain size was necessitated by the need to minimize the deleterious effects of grain boundaries evident in prior art materials. Historically, small-grained polycrystalline silicon (grain size less than 1.0 mm) has not been a candidate for photovoltaic material due to grain boundary effects. Grain boundary recombination led to degradation of voltage, current and fill factors in the solar cell. Previous models, for example Ghosh (1980) and Fossum (1980), based on recombination at active grain boundaries correctly predicted performance of historical materials. These models teach that if active grain boundaries are present, they prohibit the utilization of small grained materials in high performance solar cells.